This invention relates to the bleaching of lignocellulosic pulp. More particularly, the invention is directed at a novel chlorine-free bleaching process for such pulp. This novel process affords a sequence of bleaching stages wherein the reaction conditions of each stage are of such pH and temperature that substantially no pH or temperature adjustment is needed prior to the next following bleaching stage of the sequence. Moreover, the effluent derived from the bleaching stage itself or that washing which preferably follows each stage is suitable for use as counter-current wash water in the next earlier sequential bleaching stage in continuous bleaching processes.
Wood is composed of two main parts--a fibrous carbohydrate or cellulosic portion and a non-fibrous portion comprising a complex chemical, commonly referred to as lignin.
For use in paper-making processes wood must first be reduced to pulp, which can be defined as wood fibers capable of being slurried or suspended and then deposited as a screen to form a sheet. The methods employed to accomplish this pulping usually involve either physical or chemical treatment of the wood, or perhaps some combination of the two processes, to alter its chemical form and to give desired paper properties.
In mechanical pulping, the wood is physically ground to a high-yield pulp, most often referred to as groundwood pulp. In chemical pulping, the wood chips are digested with chemical solutions to solubilize a portion of the lignin and effect is removal. The more usual of these digestive procedures are the sulfite, sulfate or Kraft, soda and modified sulfite processes.
After the wood has been digested, the resulting material is generally a darkly colored cellulose fiber. The dark color is attributable to the fact that not all of the lignin has been removed during digestion and the remaining lignin has been chemically modified. This dark pulp is commonly referred to as unbleached pulp. It may pass directly to the paper making operation if the paper color is unimportant.
More usually, the unbleached lignocellulosic pulps are bleached to a brightness consistent with the planned utilization of the pulp, such brightness being a measure of the reflectivity of the pulp under standardized conditions. Pulp bleaching is most often a multi-stage process employing chlorine or chlorine-containing compounds such as calcium hypochlorite, sodium hypochlorite and chlorine dioxide. Exemplifying the chlorine bleaching of lignocellulosic pulps are the processes disclosed in U.S. Pat. Nos. 1,957,937; 2,975,169, 3,462,344; C. Mlakar & J. Peltonen, "Peroxide in the Semibleaching of Kraft Pulp", Paperi ja Puu, 629-638 (1968); M. Delattre, "Hydrogen Peroxide as a Bleaching Agent for Kraft Pulps", Paperi ja Puu, 117-127 (1971); and P. Christensen, "Bleaching Sulphate Pulp with Hydrogen Peroxide", Pulp and Paper Magazine of Canada, 62-66 (1971).
Although such chlorine and chlorine containing compounds have proven to be effective bleaching agents, the chlorine compounds are difficult to handle and hazardous to both personnel and machinery. The effluents from chlorine bleaching process contain large amounts of chlorides as the major by-product of the bleaching process. The chloride salts readily corrode paper-making equipment. Moreover, chloride salt concentration build-up precludes recycling in closed system operation without employing recovery operations using expensive metallurgy. Further, discard of the effluents from chlorine-based bleaching processes poses serious pollution problems.
To avoid these disadvantages, the paper industry has worked to reduce or totally eliminate chlorine containing compounds from multi-stage bleaching processes for lignocellulosic pulps. Complicating these efforts is the requirement that high levels of pulp brightness, 80% and above, are required for many of the uses for such pulps.
One method wherein the amount of chlorine containing compounds necessary to afford a high degree of pulp brightness is reduced is described by P. Christensen, "Bleaching of Sulphate Pulps with Hydrogen Peroxide", Norsk Skogindustri, 268-271 (1973). There, oxygen is employed as a first stage bleaching agent to solubilize a major amount of lignin for removal. The remaining lignin is best removed to afford a fully-bleached pulp by more classical chlorine bleaching methods but using much reduced amounts of chlorine bleachant. However, even at such reduced chlorine concentrations, the corrosive and polluting chloride salts would soon reach unacceptable levels of concentrations in closed cycle operations.
One pulp bleaching method which eliminates chlorine containing bleaching agents is disclosed by N. Liebergott, "Paprizone Treatment, A New Technique for Brightening and Strengthening Mechanical Pulps". This bleaching process, in effect a single stage, combines peroxide and ozone in a synergistic combination to brigthen and strengthen mechanical pulps. It should be noted that this process is not suited to chemical pulps, nor is it capable of achieving high pulp brightness (80%) with either mechanical or chemical pulps.
Canadian Pat. No. 966,604 discloses a multi-stage bleaching process which also wholly eliminates the disadvantageous chlorine compounds. This process is characterized by from one to three ozone bleaching stages and a final treatment with alkaline hydrogen peroxide, each stage being separated by an alkaline extraction. One such sequence may be described in the common shorthand nomenclature of the paper industry as ZEZE P.sub.alk wherein "Z" represents ozone bleaching; "E"--alkaline extraction; and "P.sub.alk "--alkaline hydrogen peroxide bleaching. In accordance with this process, the effluent from each treatment stage may be collected and recycled for use in subsequent bleaching operations, preferably at an earlier stage than that from which it was obtained. This provides a so-called countercurrent effluent flow.
Other non-chlorine bleach sequences are described by S. Rothenberg, D. Robinson & D. Johnsonbaugh, "Bleaching of Oxygen Pulps with Ozone", Tappi, 182-185 (1975)--Z, ZEZ, ZP.sub.alk and ZP.sub.a (P.sub.a --peroxyacetic acid); and N. Soteland, "Bleaching of Chemical Pulps With Oxygen and Ozone", Pulp and Paper Magazine of Canada: T153-58 (1974)--OZEP.sub.alk (O--Oxygen), OP.sub.alk and ZP.sub.alk.
However, in all prior multi-stage non-chlorine lignocellulosic pulp bleach sequences large reaction condition disparities exist between sequential adjacent stages. For example, in Canadian Pat. No. 966,604 the ozone bleaching stages are conducted at 25.degree. C. (77.degree. F.) while the following intermediate alkaline extractions are effected at about 65.degree.-71.degree. C. (150.degree.-160.degree. F.) and the final alkaline peroxide stage at about 44.degree. C. (112.degree. F.). Therefore, both heating and cooling stages must be included in any operative sequence. Moreover, the alkaline extractions are carried out in 1-2% sodium hydroxide solutions but the final alkaline peroxide stage is effected in only 0.48% sodium hydroxide. Hence, a significant pH adjustment is required. Similarly, in Rothenberg et al., "Bleaching of Oxygen Pulps with Ozone", the ozone bleaching is effected at room temperature (about 25.degree. C.) but the extraction or peroxide stages are carried out at 70.degree. C. and in "Bleaching of Chemical Pulps with Oxygen and Ozone", the oxygen stage is conducted at 110.degree. C. and the other stages at 50.degree.-60.degree. C.
Such marked variations in both temperature and pH between adjacent stages in a multi-stage bleaching process greatly disadvantage these systems. Instead significant efforts must be expended to cool or heat and acidify or basify the pulp slurry as it sequentially moves through the bleaching process. This is uneconomical from a material, equipment, and energy standpoint.